Evolution of Inner Retinal Photoreceptors: Mammals Have Lost a Melanopsin Gene

Abstract

Purpose: : Recent work has demonstrated the presence of a new functional photopigment, melanopsin (OPN4), in vertebrates. We have used a combination of in silico and classical molecular biology approaches to examine the evolution of melanopsin genes in various vertebrate classes.

Methods: : The nucleotide and protein sequences of human melanopsin (OPN4) were used as bait to probe the zebrafish, chicken and Xenopus tropicalis genomes (http://www.ncbi.nlm.nih.gov/BLAST/). Standard molecular techniques such as RT–PCR and in situ hybridization techniques were used to refine the in silico observations in zebrafish, chicken and Xenopus laevis tissues.

Results: : We have identified a novel melanopsin gene in zebrafish, chicken and Xenopus that appears to be the true ortholog of mammalian melanopsin. This novel melanopsin (which we have termed Opn4m) shares >70% amino acid identity with human melanopsin across the transmembrane domains compared with ∼58% identity to the original Xenopus melanopsin (termed Opn4x). RT–PCR analysis in zebrafish, chicken and Xenopus laevis identifies expression of both Opn4m and Opn4x genes only in tissues known to be photosensitive (eye, brain and skin). In the eye of 14 day old chickens, Opn4m is found in a subset of cells in the inner nuclear, outer nuclear and ganglion cell layers, the vast majority of which also express Opn4x. A comprehensive in silico analysis of vertebrate genomes indicates that while most vertebrate species have both melanopsin genes, chromosomal reorganisation events early in mammalian evolution have resulted in the loss of Opn4x from all members of this class studied to date.

Conclusions: : Our findings show for the first time that non–mammalian vertebrates retain two melanopsin photopigments, while mammals have just one. These data raise important questions regarding the functional differences between Opn4x and Opn4m pigments, the associated adaptive advantages for most vertebrate species in retaining both melanopsins, and the implications for mammalian biology of lacking an Opn4x ortholog.

Keywords: gene/expression • opsins • photoreceptors